Apparatus for recording sound on film



April 30, 1935. s. c. WHITMAN I APPARATUS FOR RECORDING SOUND ON FILMFiled April 18, 1,952

NVENTOR. Z.

CSM/waar e. 917/217 W V||IP Patented Apr. 30, 1935 UNITI-'Efo STATESAPPARATUS .Foa RECORDING SOUND WON FILM

stewart. c .fwhitmam Jackson" Heights, N. Y. Application April 18,1932,' Serial No. 605,993 .7"c1aims. (cuve-100.3)

The invention relatesto Vnew-"and useful improvements' in the art ofrecordingY sound 'on fiilm and :more Y:particularly to an apparatusiandmethod 1' for producing i the desired result.

VAn object of Athe invcntion1is toproducean 'apparatus for recordingsound 4on nlm VVover :the full range of audiofrequencies.

Afurther'object ofthe invention'is'to produce 'an apparatus to Yrecordsound on Aiilm in s'uch .manner that onlythe desired soundsrresultfuponreproduction.

Another object of the invention is ilto fprovide fan apparatus;` thecomponent 1 partsof which kcan 'be 5 fully supervised and .controlled at`all ftimes 'during operation. f l

Still? another :obj ect7 'of the' invention.; isito-v pro- -videanapparatus Whichmakespossible the-:sav-

ing' ofiilmv in'soundrrecording.

'A lstill "further .fobjectzof :the 'invention i is, `to :produce an:apparatus which :cani beioperated` directly 'fromianalternating':current 'p ower s`upply.

'With these: and` numerous?otherobjects in view, my invention consistsinthe-novel features of construction; combination,v and'4F arrangementof: parts fas will be hereinafterreferredztoz and more g particularlypointed-1out -in .the :specification and 'In' the drawinfgfiorming aApart voi" this. application: 1 v

The ligurev isca schematic diagramshowing the :arrangement and coupling.among the various component *elements of l my improved apparatusutilized in my system of 1 recordingsound "onf fil-m.VIn-fdescribingtheiinvention, Ishall refer to lthe drawing in -.-which'the ynumeral I designates -a Imicrophone .2 is l:an audio -famplier--for f the microphone signals ;V :3 is `anf instrument .forregulatingthe amount of amplificationto be obtained by: means vof l 2. .The inumeralwl designates ade- `vice -vwhich supplies, a .continuous -visualindication of the power level measured in decibels; The numeral 5designatesqa small( .output 1amplierfeeding the loudspeaker-6. f'I'he-signaloperating-5 isvrtapped from 2 and isfreproduced-in .6 Ltherebyaffording .the f operator a= continuous audible checkonfsound qualityand volume. Th volume r'level of monitor 5v sVa-riable.

.-The .numeral 'I designates faccupling .stage through Vwhich theamplified :signal :from s2 .is conducted totheimodulator Vstage .8.kA:'Illenulmeral 9 designates Va1'ractin-frequency vacuum 'tubecscillatonthe output of kwhich is introduced into f the r modulator 8Aand is therein .modulated .byftheaudio signal introduced from'i.. Y Thenumeral :IIS designatesfa radio frequency amplifier. The modulatedradiofrequency signal'from 8 is amplified in Ill andfed to the recording'lamp 'I I. The ynumeral, I2 designates 'a potentiometerfbymeans.of-whichthe output of' I D'maybe tapped to 'operateithe 'earphones I3thereby affordingthey operator means of checky ing the signal,whenfandjifdesired, arriving at the recording lamp II. "The numeral I4:designates al device whereby the grid bias ofthe modulator tubei'n maybe automatically determined.

and controlled by the strength/of' the audio signal in 2, :asvhereinafter and more particularly set `'forth.-l .-1 V The numeral I 5designates a rectier'and lter system' forc supplying the 1 necessarypotentialsvto r'the-various component parts, thepower being de'- rivedfrom.' f the alternating current source I6. 'The connecting linesindicate coupling -means amongthe-v'arious component parts.

' I shall nowmore particularly'l describe the operation `cfjmyinvention. There are two general methods used `at present in :recordingsoundv on lnlm: 'i the light-valve method and -thefglow l lamp method.In" the' light-valve method, f light '"frcm aconstant-source falls-uponthe lm after ybeing modulated bypassing through a mechanical 'lightshutter. I'I'his vmechanical shutter isoperated 'electrically vby'ftheaudio. signal.` In the :glow lamp method,1a constant y'direct currentactu- 'ates -a Ygaseous-conducticn lamp :and Vthe steady lightemittedthereby'is modulated by superimposing` the Yaudio signal voltagethereupon.l

'Inherent faults are recognized in Vthese 'systems. The 'frequency rangeofthe light-valve system is limited Vdue tothe: inertia ofthe movingmechanism. In the case of thev glow lamp' meth- .o'd'the'steady-D C.exciting current Vproduces vexposure. on ythe .lm with nc signalmodulation.

On `the positive .print the unmodulated track transmitslightand,groundnoises result upon reproduction. The `irregularities in thebattery circuit actuating-the lamp. areyreproduced on the :Iilmxrecordand further-aggravate the-ground 'Ihe extinction voltageof a glowdischarge lamp is considerably lowerthan-'the ignitionfvoltage. Thus ifthey audio-modulating' voltage 'rises suiirciently thigh the Vglow lamp'is extinguishedv and `the modulating voltage must be loweredconsiderably before .the lamp -re1ights.1 .overloading of this kindoccurs frequently in recording and this o llagvin the glowlampconstitutes ay serious dra back. Y Y v It has been proposed toutilizeradio-,frequency `Aalternatingcurrent'to activate the glcwpilamp. 55

ka separatev oscillator.

'and output impedances. In ,'tortlon is introduced. 1 l

be impressed across the lamp which would be less.

than Ydirect current of corresponding voltage.

Some improvement in ground noise is thus ob'-" tained.V A

I propose to describe here a method ofglow Vlamp recording and anapparatus which eliminates all the undesirable features. The sound to berecorded is transmitted through a microphone and amplified. Theresulting signal is used-to modulate a radioffrequency carrier generatedin I have found a frequency of `about 200 K. C. to perform best. Itisim- 'portant that this modulation be accomplished through the medium ofa modulator tube. Another important feature in this modulation processis vthe utilization -of Va coupling tube stage between the audioamplifier and the modulator stage for the purpose of Vproperly-matchingthe input this manner, no dis- By using a separate modulatortube stagean essential improvement can be introduced. n' ,The grid bias on themodulator Atube is determined so that after subsequent amplification,which will be described later, the resultant, unmodulatedradio-frequency voltage impressed on the recordy ing lamp is justsufficient to ignitej theflamp. A

imaintained at a fixed value. of the automatic bias changing device justde- 'extinction voltage of the recording lamp. other words,'for thepurpose of this discussion,

small portion of the Yaudio modulatingA signal is tapped from theVoutput of thev audio amplifier.

This signalis amplified and rectified and impressed upon the modulatortube grid bias supply in such fashion that this variable impressedvoltage acts in an opposite direction to the grid bias voltage and tendsat all times to reduce; the grid bias voltage. This reduction in the`grid bias voltage results in higher radio frequency voltages Vbeinggenerated in the plateK circuit of the modulator tube. With the gridbiaschosen originally, the recording lamp is justbrought to the ignitionpoint. It can be VseenY that even the smallest audio-frequency signalvoltageentering the modulator'stage would extinguish the recording lampif the grid bias applied to the modulator tube were However, by meansscribed any 'audio signal must increase the radio frequency carrier.voltage in the modulator tube.

By adjusting the amplification of the tapped audio 'signal properly, thebias is varied so that the carrier voltag'eis just suflicient forcomplete modulation at all times. By complete modulation Vis meant thatdegree of modulation which varies the radio frequency carrier voltagealmost to the In the extinction voltage of the lamp is considered thezero limit for modulation.A This statement'takes into account the factthat the modulated carrier is later amplified before being impressedupon rthe recording lamp. Y

To summarize, then, we have the followingconditions during operation.The primary advantages of using ra'dio'frequency excitation oftherecording lamp are obtained.`In addition, the

unmodulated excitation current results in light of minute value, thusproducing practically no sound track without modulation. With modulationof the radio frequency current taking place, practically all the lightemitted by the recording lamp is a result of the audio signal. On thepositive print therefore the unmodulated track is opaque and themodulated track transmits in direct proportion to the recorded sounds.In the reproducing system therefore no light can activate the photocellexcept that due entirely to the desired recorded sound. Reproductionresults which is entirely free from extraneous noises.

From actual experience in the operation of my apparatus, I find thefollowing ratings preferable.

The audio amplifier has an overall gain of approximately decibels. Theoscillator used for generating the radio frequency excitation currenthas a power rating of about '7.5 watts. A 50 watt modulator i! used andfeeds into a 50 watt amplifier stage. I haverfound that this high poweris necessary in order to properly match all Vthe various componentswithout distortion and still deliver sufficient power to the recordinglamp. The amplifier stage following the modulator is necessary intransforming the modulated radio frequency output of the modulator tothe proper voltage for transmission to the recording lamp. ThisAamplifierrstage also serves as a `coupling stage between-the modulatorand the recording lamp wherein the proper impedance match canbe'accurately accomplished. Y V- By .arrangingthe apparatus with theaudio amplifier, oscillator, *modulaton modulator bias control, andradio frequency amplifier, as separate units, complete control andaccurate adjustment of every operation is obtained. The audioamplificationcan be monitored and varied at. will. `The frequency of theoscillator can be raised and lowered. Thecoupling'to the modulatorcan-*be adjusted? to: produce'any radio frequency' voltage from'zero to`maximum at the recording lamp.

Vabout 1-/50 second. Thus, at no time can any main signal frequencygreater than 50 cycles arrive at the modulator grid before the properadjustment of grid bias has been made. As a. result of this, a widerange of signal intensities can be handledvby the recording lamp with nopossibility 'ofextinction due'to'overload and with noy possibility ofunnecessary exposure due to unmodulated light. Y Y Y I have successfullyrecorded frequencies as high 'as fourteen thousand cycles-the practicallimit of the amplifying equipment-with the apparatus o disclosed'. Theserecords show a clear cut delineation between adjacent cycles'under themagnifying glass. Using the high power available in the presentinvention' in connection with aA specially constructed radio-frequencyrecording lamp, which is described in my 'co-pending application, SerialNo. 605,990, filed of an even date herewith, using a rspeciallyconstructed optical system which is described in my copendingapplication, Serial No. 605,992, led of' an even'date herewith, andutilizing the maximum photo- -graphic contrast obtained by my automaticmodu- .lator grid bias control, excellent sound records can i be made`-at f- :film` speedsv of 60. feet kper i minute. `Since'this film'speedis. satisfactory for pictorial photography, atremendous savingy inlmstock'is .-zmadepossible. The present sound recording filmspeed-isusually 90 feet per minute,v

In the presentinvention, allV power. used by: the tubesmaybe suppliedfrom a. rectified and-filtered alternating current line source.ll'Iliis. is a great eaid.. in preservinguniformity Y*since voltages-andcurrents may' be checked .andfeasily adjusted-at vall times'xwithpnodanger of power '.diminutionror failure-such asisxfrequently.encountered with fbatterypowerg.

iWhile I have particularly 'described V:the elements best.- adapted` toperform the; functions: set Sfor.th'it' is. obvious that various changesin form, proportion and in the. minor` detailsV of. construc- Ationmaybezresorted'to,.without departing from '.the; spirit vorfsacriicingany of fthe .principles of the invention. f f

`-IIavingfthusdescrbed thel invention, what is claimed is:

1. In an apparatus for recording sound on film, means for providing ahigh frequency current, a sound pick-up device, an* amplifier therefor,a Vseparate modulated tube stage wherein the high frequency current ismodulated by the output of the sound amplifier, a source of fixed gridbias potential for said modulator tube stage, means for tapping aportion of the sound amplifier output current, means for rectifying andamplifying the said tapped current, means controlling said amplificationto obtain a balanced relationship of substantial equality between thelast mentioned current and the rectied output current of the soundamplier, and means for impressing the balanced tapped currentsubtractively upon the fixed grid bias potential of the modulated tube.

2. In a sound recording system using a glow lamp wherein substantiallyno light is emitted from said glow lamp when no sounds are delivered tothe input of the system, in combination, a microphone, a speech inputamplifier, a source of radio-frequency current, a separate modulatortube, the output of said source of radio-frequency current beingdelivered to said modulator tube, the output of said speech inputamplifier being delivered to said modulator tube for modulating Saidradio-frequency current, a recording glow lamp, means for delivering theoutput of said modulator tube to said glow lamp, means for applying aXed grid bias potential to the grid of said modulator tube, andrectifying and amplifying means connected between said speech inputamplifier and said modulator tube for applying to the grid of saidmodulator tube a varying unidirectional potential varying in magnitudewith the output of said speech input amplifier and in opposition to saidxed bias potential, said means for applying a xed grid bias potential tothe grid of said modulator tube being primarily adjusted so that when nosound energy is applied,to said microphone and speech input amplier theenergy delivered to said glow lamp from said modulator tube is justsufhcient to ignite said glow lamp.

3. In a sound recording system using a glow lamp wherein substantiallynolight vis emitted from said glow lamp when no sounds are delivered tothe input of the system, in combination, a microphone, a speech inputamplifier, a source of radio-frequency current, a separate modulatortube, an adjustable coupling tube unit connected grid of said: modulatortubel being. primarilyv adjustedfso that-when no soundt 'energy isapplied .to:.-sai'drnicrophone andspeech input amplifier, the lenergydelivered to said yglow lamp.' from said ;modulato'rftube is -justsuiiioient to vignite said glow lamp. w

4. -In-aa.sound;recording.system using va glow lamp 1 wherein.substantiallym light s .is emitted' from saidvglo'w Vlampiswhen nosounds 4are deliverediA to; the. inputI ofthe system,; injco-mbination,a microphone, a speech input amplieniafsource of. radio-frequency.current, a separate modulator tube, the output of said source ofradio-frequency current being delivered to said modulator tube, theoutput of said speech input amplifier being delivered to said modulatortube for modulating said radio-frequency current, a recording glow lamp,means for delivering the output cf said modulator tube to said glowlamp, means for applying a fixed grid bias potential to the grid of saidmodulator tube, and rectifying and amplifying means connected betweensaid speech input amplifier and said modulator tube for applying to theYgrid* of said modulator tube a varying unidirectional potential varyingin magnitude with the output of said speech input amplifier v and inopposition to said xed bias potential, said means fory applying a fixedgridbias potential to the grid of said modulator tube being primarilyadjusted so that when no sound energy is applied to said microphone andspeech input amplifier, the energy delivered to said glow lamp from'said modulator tube is just sufficient to ignite said glow lamp, thelevel of amplification of said rectifying and amplifying means being soadjusted that the potential delivered to said glow lamp v at no instantgoes below the extinction potential.

5. In a sound recording system using a glow lamp wherein substantiallynol light is emitted from said glow lamp when no sounds are delivered tothe input of the system, in combination, a microphone, a speech inputamplifier, a source of radio-frequency current, a separate modulatortube, the output of said source of radio-frequency current beingdelivered to said modulator tube,

a recording glow lamp, means for delivering the'r output of saidmodulator tube to said glow lamp, means for applying a fixed gridbiaspotential to the grid of said modulator tube, rectifying andamplifying means connected between said speech' input amplifier and saidmodulator tube for applying tothe grid of said modulator tube a varyingunidirectional potential varying in magnitude with the output of saidspeech input amplifier and in opposition to-said fixed bias potential,and electrical impulse delay means connected between the output of saidinput speech amplier and the input of said modulator tube forV impartinga slight time delay to the output directly delivered to said modulatortube from said input speech amplifier with reference to the poten- Ymeans for applying a fixed grid bias potential to the grid of saidmodulator tube being primarily adjusted so that when no sound energy isapplied to said microphone and speech input amplier the energy deliveredto said glow lamp from said vmodulator tube is just sufficient to.ignite said glow lamp. Y

V6. InV the recording of'sound on lm employing a recording glow lamp anda separate modulator tube, the methodofvrecording sound with minimumbackground noise which consistsl in applying to the grid of saidmodulator tube a xed Agrid bias potential, the level of amplification of"said'portion of the speech input electrical energy being such .that thepotential delivered to saidY glow lamp at no instant goes below theextinction potential. y

Vby saidv rectifying and amplifying means, said V!7.'In;the recording'ofsound Aon illm employ- 'ing a recording glow lamp and a separatemodulator tube, the method of recording sound with minimum backgroundnoise which consists in applying tothe grid of said modulator tube afixed grid bias potential of such magnitude that said glow lampV justignites when no speechginput electrical energy is deliveredto saidmodulator tube, amplifying'rectifying and applying to the grid ofsaidmodulator' tube a portionrofwthe speech input electrical energy inopposition` to lsaid xed grid bias potential, the levelv of amplicationof `said portion of the speech input electrical energy being such thatthe potential delivered to said glow lamp at no instant goes f below theextinction potential, and imparting a `slight 'time delay to the speechinput 'electrical Venergy .directly delivered to the input of saidmodulator tube *with reference tothe amplied and rectied portion of thespeechinput electrical energy applied to the grid of said modulatorktube.-

- 1 fSTEWART C. 'WI-HTMAN.

